This function is used to asynchronously read a certain number of bytes
of data from a stream. The function call always returns immediately. The
asynchronous operation will continue until one of the following conditions
is true:

The supplied buffers are full. That is, the bytes transferred is equal
to the sum of the buffer sizes.

An error occurred.

This operation is implemented in terms of zero or more calls to the stream's
async_read_some function, and is known as a composed
operation. The program must ensure that the stream performs
no other read operations (such as async_read, the stream's async_read_some
function, or any other composed operations that perform reads) until this
operation completes.

The stream from which the data is to be read. The type must support
the AsyncReadStream concept.

buffers

One or more buffers into which the data will be read. The sum of
the buffer sizes indicates the maximum number of bytes to read from
the stream. Although the buffers object may be copied as necessary,
ownership of the underlying memory blocks is retained by the caller,
which must guarantee that they remain valid until the handler is
called.

handler

The handler to be called when the read operation completes. Copies
will be made of the handler as required. The function signature of
the handler must be:

voidhandler(constboost::system::error_code&error,// Result of operation.std::size_tbytes_transferred// Number of bytes copied into the// buffers. If an error occurred,// this will be the number of// bytes successfully transferred// prior to the error.);

Regardless of whether the asynchronous operation completes immediately
or not, the handler will not be invoked from within this function.
Invocation of the handler will be performed in a manner equivalent
to using boost::asio::io_service::post().